Method and device for polarizing ferroelectric materials

ABSTRACT

Method and means for polarizing ferroelectric materials up to a predetermined polarization level include the application of an electric field E to these materials. According to this method the ferroelectric material is subjected to an alternating electric field E, the frequency of which ranges from about 0.001 to 1 Hz, and which is made to increase gradually and in a cyclic way between 0±E N , E N  being slightly in excess of the coercive force E C  of said material. Simultaneously, the intensity i of the current traversing the material (2) is measured as a function of the applied field (E) using a unit of visualization, until a stable curve i=f(E) is attained. The invention is particularly suitable for obtaining a stable polarization of ferroelectric polymers, copolymers, crystals, and polycrystals, with the objective of using the piezoelectric and/or pyroelectric properties of these materials.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to methods and means of polarizing ferroelectricmaterials, particularly crystals, polycrystals, polymers or copolymers,such as, for example uniaxially or biaxially stretched polyvinylidenefluoride.

As soon as the aforementioned materials are polarized, and taking intoaccount their remanent polarization, they are known to exhibitpieozoelectric and/or pyroelectric properties which make them suitablefor industrial applications. The piezoelectric properties of thesematerials allow them to be used as emitting transducers for loudspeakersand earphones, hydrophones, echo sounding in media such as air, water,biological tissues, or as pressure transducers for microphones,chronographs for shock waves, pressure tranducers for shockwaves asradiation pressure transducers, and also as probes for echography andhydrophones.

The range of practical use of the pyroelectric properties includestemperature measurements, detection of hot points detection ofintruders, and recording of infrared pictures. Within this range ofpractical use the ferroelectric materials are generally very thin films,the thicknesses thereof ranging from a few micrometers to onemillimeter.

For the various industrial applications considered herein, both thepiezoelectric and pyroelectric coefficients of the materials used mustbe known. Simultaneously, these coefficients must be reproducible in themanufacturing process. These coefficients are directly dependent,however, on the remanent polarization of the above-mentioned materials.

R. Hasegawa et al. (J. Polym Service A, 8, 1970), F. Micheron (Reviuetechnique Thomson CSF, volume 11, 3, 1979) and P. E. Bloomfield et al.(Naval Research Reviews, volume 31 No. 5, 1977) reported on methods andtechniques for polarizing ferroelectric crystals, polycrystals,polymers, and copolymers. The specimen to be polarized is usuallysubjected to an electric field applied at room temperature or at atemperature level being in excess of or close to the Curie point. Theelectric field applied causes the polar axis, carrier of a permanentdipole, to be oriented in a preferred direction which is the closest oneto that of the electric field. After the electric field has been removedthe poled ferroelectric element exhibits a stable remanent polarizationat room temperature.

Various techniques are used for the application of this electric field,examples are the method based on simple electric contacts, the coronadischarge method, the plasma technique, and so forth. Because of theirsimplicity all these methods are undoubtedly of great advantage, butthey do not allow one to get insight into the state of polarization ofthe material under study, that is, into the part by volume of poledmaterial, the homogeneity of polarization on the surface and within thematerial, and the remanent polarization achieved. On the latter,however, depend the piezoelectric and pyroelectric coefficients whichare proportional to this polarization. This is especially true forferroelectric polymers and copolymers.

It is possible to measure the remanent polarization, but only veryapproximately, by measuring the total electric charge released in thecase of the pyroelectric depolarization. However, the properties of thematerial under study set limits to such measurements which, in addition,are not accurate enough because the material heated up undergoesdielectric losses too high to be accepted.

Also known is the method reported by Sawyer and Tower (C. B. Sawyer andC. H. Tower)--Physical Review, volume 35, 269, 1930), modified by J. C..Hicks (J. C. Hicks, T. E. Jones, Ferroelectrics, volume 32, 119-126,1981), which allows the material to be polarized as follows. Asinusoidal or triangular electric field is applied (±E [MV/cm]) and theelectric induction D is recorded as a function of the applied electricfield E. The curve plotted in this way has the shape of a hysteresisloop D=F(E), but does not indicate the homogeneity of both the remanentand instantaneous polarization of the material because the parameter Dused in the measurements takes into account the effects resulting fromthe ion currents or from the space charges as well as the capacitiveparasitic effects emanating from the dielectric element considered. Thepolarization achieved in this way and the remanent polarization affectall the piezoelectric and pyroelectric properties of the material.

SUMMARY OF THE INVENTION

One of the objects of this invention is to overcome the difficultiesinherent in the aforemented methods by presenting a method which allowsferroelectric materials to be poled at a level such that the remanentpolarization of the material will be reproducible and stable.

A further object of this invention is to measure the remanentpolarization such that this material concerned can be characterized viathe measured results.

Another object of this invention is to achieve remanent polarizationwhich is homogenous within and on the surface of the materialconsidered.

The invention for poling ferroelectric materials up to a predeterminedlevel of polarization includes the application of an electric field E tothese materials.

According to a feature of this invention the ferroelectric material hasan alternating electric field E applied thereto with the frequency ofthe field ranging from 0.001 to 1 Hz, by making this field increasegradually and cyclicaly between O and a value ±E_(N), with E_(N) beingslightly in excess of the coercive force E_(c) of the materialconsidered.

According to another feature the current i traversing the material issimultaneously measured as a function of the field E by using a unit ofvisualization, until a stable curve i=f(E) is achieved.

In this way a low-frequency alternating electric field can be applied tothe ferroelectric material, the amplitude of which increases veryslowly. As a consequence, the ions and space charges are drained towardthe electrodes and after a certain time period--for a given electricfield--the polarizing current ip is kept constant at a level whichcorresponds to the oriented polar crystallinity of the material.

Surprisingly, it has been found that by processed as outlinedhereinabove, it was possible to obtain a stable and reproduciblepolarization, which remained unchanged with time.

According to a preferred version of the invention, the electric field Eis made to increase gradually by approximately 0.05 MV/cm/min.

This gradually increase of the electric field to the value E_(N)suffices for obtaining convenient and reproducible characteristics ofthe material considered.

According to another aspect of this invention, the device for polingferroelectric materials up to a predetermined level of polarization ischaracterized in that it comprises a source of high voltage alternatingcurrent as well as components for applying this voltage to the materialto be poled, for altering both the level and frequency of the voltageapplied and hence, for altering the electric field E applied to thematerial, and for substracting the capacitive component i_(C) and theresistive component i_(R) from the current i traversing the materialconsidered.

The current i passing through the material to be poled is defined by thefollowing relationship as a function of the field applied:

    i=ε(dE/dt)+(dP/dt)+(E/R)                           (1)

where

E is the electric field applied

P is the polarization of the material

ε is the permittivity of the material

t is time

R is the internal resistance of the material with the electric fieldapplied.

By substracting the current i_(C) =(dE/dt) due to the capactivecomponent as well as the current i_(R) =E/R due to the resistivecomponent, the device allows the pure cycle i_(p) =(dP/dt) to beobtained as a function of the electric field applied. Thereafter via anintegration performed as a function of time, the device allows thehysteresis loop of polarization to be obtained directly as a function ofthe electric field applied. Thus, the development and the subsequentstabilization of this loop can be observed as the field strengthincreases.

The features which characterize the invention are pointed out in theclaims. Other objects and advantages of this invention will becomeapparent from the following detailed description when read in light ofthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a device embodying the invention.

FIG. 2 is a curve illustrating the variation in intensity of current itraversing the specimen to which a sinusoidal electric field isgradually applied.

FIG. 3 is the stable curve i=f(E) plotted after the electric field hasbeen applied.

FIG. 4A is the curve i=f(E) obtained at a certain stage of the process,on which the capacitive component as well as the resistive component ofthe current become apparent;

FIG. 4B is the polarization P=f(E) and the current i=(dP/dt) after thecapacitive component and the resistive component have been subtracted.

FIG. 5 is an example for the curves (P) and (i) which are analogous tothose plotted in FIG. 4B and which corresponds to a particular case ofthe ferroelectric material considered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the device for poling ferroelectric materials includes asource 1 which delivers sinusoidal high voltages and serves to apply anelectric field E to a specimen of ferroelectric material 2 placedbetween two plane electrodes 3a and 3b. A connection or line 4 traversedby a current i connects the terminal 5 of source 1 and the electrode 3a.The electrode 3b is connected to the input 6 of an amplifier 7 which isprotected from overvoltages by a relay, not shown in FIG. 1, which isdriven by the aforementioned amplifier 7. A second input 8 of theamplifier 7 at ground potential. The terminal 9 of source 1 is connectedto ground potential and completes a first circuit 5, 4, 7, 9, 10.

The electrodes 3a and 3b used for the application of the field E areplaced in a thermoregulated chamber 10 indicated in FIG. 1 by a dottedline, if the device is to be operated at temperatures other than roomtemperature. According to one embodiment they serve to transmit highpressures--200 bar, for instance--to the sample to be poled in order tomaintain its planeness or flatness during the process of polarization.In effect it seems that as soon as the field is applied causing thepolarization to attain high levels, phenomena are generated such as astrong motion of the crystalline chain, of the ions, and of the spacecharges within the material, which can result in its deformation. Forindustrial application of these poled ferroelectric materials, it isessential that their geometric characteristics--in particular theplaneness or flatness and the parallelism of their principalsurfaces--present a very high degree of accuracy, within 1 μ, and thatthis accuracy be maintained.

The current i traversing the specimen can be displayed and measured withthe aid of an oscillograph 36 whose input along the y-axis is connectedto the output 11 of the amplifier 7, while the input along the x-axis isconnected to the source through its terminal 15, which delivers anattenuated voltage as illustrated below.

The output terminal 11 of the amplifier 7 is connected to an input 12 ofan adder 13 equipped with three input ends. The second input 22 of thesethree input ends is one of the two end parts of a connection 14. Theother end part is connected to a third terminal 15 of source 1 deliversa signal whose amplitude is attenuated with respect to that applied tothe terminal 5, by a factor 10³, for instance. The terminal 15 isconnected to the input 16 of a voltage amplifier and inverter 17, theoutlet terminal 18 of which is connected to the input 19 of a variableor controllable amplifier 20. The output 21 of this amplifier 20 isconnected by a connection 14 to the second input 22 of the adder 13. Theexclusively resistive branch circuit (19, 22) is traversed by a currentthat can be adapted, via the amplifier 20, to the value of the resistivecomponent i_(R) of the current i. Elements 17 to 22 form a secondcircuit.

A third circuit is connected in parallel across the terminals 19 and 23of the second circuit. It includes a π/2 phase shifter 25, which isconnected to the terminal 19 through the input terminal 26. The outputterminal 27 of phase shifter 25 is connected to the input 28 of acontrollable or variable amplifier 29, the output terminal 30 of whichis connected to an input 31 of a second adder 32 by a connection 24. Theadder 32 is equipped with three inputs, the second input 33 of theseinputs being connected to the third input 23 of the adder 13. Thecircuit serves both to isolate and display the capacitive componenti_(C) of the current, which is adjusted by means of the variable orcontrollable amplifier 29.

A fourth circuit is the circuit which allows the polarization curve tobe displayed as a function of the field applied. It includes aconnection 44, a plotting table 46, the input of which along the y₁-axis is connected, via an integrating circuit 47, to the input 45 ofthe second adder 32, while the input along the x₁ -axis is connectedacross the terminal 15 of source 1.

A fifth circuit allows the compensated current i_(p) to be displayed asa function of the field applied. The fifth circuit includes a connection54, a plotting table 55, the input of which along the y₂ -axis isdirectly connected to the output 45 of the second adder 32, while theinput along the x₂ -axis is connected to the same terminal 15 of source1.

In the following, the mode of operation of the device just describedwill be illustrated and the method according to this inventionexplained.

In order to pole the ferroelectric material 2 up to a predeterminedlevel of polarization P, a sinusoidal electric field E is applied tothis material by means of the high voltage source 1. This sourcedelivers a voltage at frequencies ranging from 0.001 to 1 Hz.

According to this invention this voltage is made to increase graduallysuch that the electric field E increases itself between 0 and +E_(N),with E_(N) being slightly in excess of the coercive force EC of thematerial considered. The increase is of the order of 0.05 MV/cm/min.

For example, this value of the field E_(N), which slightly exceeds thecoercive force of the material, attains 1 MV/cm for a biaxiallystretched PVF₂ -polymer, 0.5 MV/cm for a uniaxially stretched PVF₂-polymer, and 0.01 MV/cm for a polycrystalline material of the PZT type(lead zirconate titanate).

Simultaneously the amplitude i of the current i traversing theferroelectric material 2 is measured as a function of the field E bymeans of an optical apparatus, for instance an oscillograph, until astable curve i=f(E) is obtained.

With the increase of the field E and for a value E₁ of this field, itwill be observed first that the current i follows a stable cycle C₁ (seeFIG. 2). If the field is made to further increase slowly to a value E₂,the curve of the current i leaves the cycle C₁ and follows a secondcycle C₂ which, in turn, is stable. If the field continues to increaseand reaches a given value E_(N), which is slightly in excess of thecoercive force of the material, the latter has undergone a stable andreproducible polarization (see curve C₃ in FIG. 3).

Thereafter the field E is made to increase gradually by about 0.05MV/cm/min until it reaches its maximum value E_(S) which is slightlybelow the disruptive strength of the material. For the sake of examplethis disruptive strength attains 400 volt /μ, in the case of a biaxiallystretched PVF₂ -polymer.

The curve C₃ corresponding to the field E_(N) may present two smallindents (a, b) which are positioned almost symmetrically with respect tothe origin of the co-ordinates. These indents vanish as soon as thefield E_(N) is made to increase to a level slightly in excess of theaforementioned velocity, i.e., increases in the field in relation totime, and chosen as a function of the application desired (dotted partsa₁, b₁, in FIG. 3).

If the ferroelectric material 2 is a polymer or a copolymer, theoperating temperature within the chamber 10 is permanently maintained ata level which is below or at most equal to the working temperature ofthe material under study.

In the case of crystals of polycrystals this temperature is permanentlykept below the Curie point of these materials.

In general it is of advantage to operate at a temperature level whichapproaches the room temperature. A slight temperature rise, however,facilitates the migration of the ions and space charges.

In addition, the device shown in FIG. 1 allows one to measure theintensity of the polarization current transversing the material 2cleared of both its resistive component i_(R) and capacitive componenti_(C). The current i which traverses the first series circuit 4 and thematerial 2, the value of which is given by the above-mentionedrelationship (1), is added, via the adders 13 and 32, to the currentsi_(R) and i_(C) of the second circuit 14 and of the third circuit 24,these two currents thereby being generated by the attenuated voltagesource 1, the polarization direction of which has been reversed by thereverser or inverter 17. By means of the controllable amplifiers 20 and29, the intensities i_(R) and i_(C) are adjusted as a function of thecharacteristics of the material to be polarized (permittivity, internalresistance, and so forth). Thus, one obtains, in the fifth circuit,solely the polarization current i_(p) such that i_(p) =dP/dt, and theplotting table 55 displays the cycle of this current i_(p) as a functionof the electric field i_(p) =f(E).

After the integration performed as a function of time by the integrator47 of the fourth circuit 44, the same current is applied to the plottingtable 46 which displays the hysteresis loop of polarization P as afunction of the field applied P=f(E).

The current i traversing the material 2 is shown in FIG. 4A via thecurve C₄ as a function of an applied field E₄. The resistive componenti.sub. R of this current i is shown in a well-known manner by adash-and-dot line passing through the origin of the co-ordinates, whileits capacitive component i_(C) is represented by a dotted ellipse.

The polarization current i_(p) is represented in FIG. 4B after havesubtracted both its resistive component and capacitive component fromthe current i. This curve is drawn by the plotting table 55 in the wayalready outlined hereinabove.

FIG. 4B also shows as a dotted line, the polarization curve P of thematerial plotted as a function of the applied field E. This curve wasdrawn with the help of the plotting table 46.

FIG. 5 shows, as an example, the curves indicating the variation of thecurrent i_(p) as well as that of the polarization P as a function of thefield (E) for a biaxially stretched PVF₂ -sample in thickness of 26 μmwith a surface equalling 1.89 cm². The applied field (E) is given inMV/cm, the (i) in μA, and the polarization (P) in μC/cm² (the requiredvoltage of the field attains 9.6 kV for a thickness of 26 μ of thematerial). The residual remanent polarization (P_(r)), in the case of azero-field, equals 8.3 μm/cm² in the example given.

Thus, the ferroelectric material is seen to undergo a stable, uniform,and homogeneous polarization on the surface and inside, which isperfectly reproducible even if the polarization remains below or equalsthe saturation level.

Finally, it should be noted that according to this invention, the methodallows one to demonstrate that some polymeric materials considered to bepolarizable are really nothing else than electrets.

This invention is obviously not limited to the aforementioned examples.Other embodiments of this invention may occur to those skilled in theart.

Thus, the alternating field E applied could be triangular and notsinusoidal, the essential thereby being that the frequency range liesbetween 0.001 and 1 Hz, and that the ferroelectric sample undergoes aslow and gradual increase of the electric field applied, which allowsthe ions and space charges to be drained toward the electrodes.

What is claimed is:
 1. The method of polarizing ferroelectric materials,having a coercive force E_(C) up to a predetermined polarization levelcharacterized by the steps of:(a) applying to the ferroelectric materialan alternating electric field E whose frequency ranges from 0.001 to 1Hz, while increasing the field gradually between 0 and ±E_(N), whereE_(N) is slightly in excess of the coercive force E_(C) of the material;(b) simultaneously measuring the intensity i of the current traversingthe material as a function of the field E using a display unit until astable curve i=f(E) is achieved.
 2. The method according to claim 1,wherein the electric field E is incresed gradually at about 0.05Mv/cm/min until the curve of the current i traversing the materialbecomes stable.
 3. The method according to claim 1, wherein for amaterial having a ferroelectric polymer or copolymer, the temperature ismaintained at a level which is below or at most equal to the workingtemperature of the ferroelectric polymer or copolymer.
 4. The methodaccording to claim 2, wherein for a material having a ferroelectricpolymer or copolymer, the temperature is maintained at a level which isbelow or at most equal to the working temperature of the ferroelectricpolymer or copolymer.
 5. The method according to claim 1, wherein for amaterial having ferroelectric crystals or polycrystals, the temperatureis maintained at a level which is below the Curie point of thesematerial.
 6. The method according to claim 2, wherein for a materialhaving ferroelectric crystals or polycrystals, the temperature ismaintained at a level which is below the Curie point of these material.7. The method according to any one of claims 1 to 6, wherein increasingthe electric field includes increasing the field to obtain a curve i₁=f(E₁) corresponding to a value E=E₁ of the field, and furtherincreasing the electric field E by about 0.05 MV/cm/min to a value E₂exceeding E₁ and to obtain a new and stable curve i₂ =f(E₂), andcontinuing to increase the field E until the saturation level of thematerial to be poled is attained and to obtain a stable curve i_(s)=f(E_(s)) corresponding to a value E_(s) of the field E, where E_(s) isslightly below the disruptive strength of the material.
 8. The methodaccording to any one of the claims 1 to 6, wherein measuring the currentincludes substracting from the current i =f(E) its capactive componentas well as its resistive component, and integrating the result as afunction of time to obtain the hysteresis loop of polarization P of thematerial directly as a function of the electric field E applied so thatP=f(E).
 9. The method according to claim 7, wherein measuring thecurrent includes substracting from the current i=f(E) its capactivecomponent as well as its resistive component, and integrating the resultas a function of time to obtain the hysteresis loop of polarization P ofthe material directly as a function of the electric field E applied sothat P=f(E).
 10. The method according to any one of claims 1 to 6,wherein a pressure of approximately 200 bar is applied to theferroelectric material in order to maintain the planeness of thesurfaces of this material.
 11. The method according to claim 7, whereina pressure of approximately 200 bar is applied to the ferroelectricmaterial in order to maintain the planeness of the surfaces of thismaterial.
 12. The method according to claim 8, wherein a pressure ofapproximately 200 bar is applied to the ferroelectric material in orderto maintain the planeness of the surfaces of this material.
 13. Themethod according to claim 9, wherein a pressure of approximately 200 baris applied to the ferroelectric material in order to maintain theplaneness of the surfaces of this material.